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When moved down from Application layer through each layer of OSI model at sending computer, layer protocols add some form of information as header. Datalink layer adds a trailer also. Finally, the data is placed as stream of bits over network cable wire.
When the data is received at the receiving computer, as stream of bits from wire, each layer reads corresponding information from its own header and use it for processing incoming data. After processing at each layer, protocols at each layer removes its header/trailer and the data packets are passed to the next high layer. Finally, the data sent from the sending computer is passed up to the Application layer at the receiving computer.
The packing and unpacking of data at different layers of OSI model is called as Encapsulation and Decapsulation.
Following image shows how data from Application layer is moved down through different layers of OSI model at sending computer and moved up through different layers of OSI model to reach Application layer at receiving computer.
